Steel Cord Belt and Splice Construction Modernizing Their Specifications, Improving Their Economics

20/12/2015 Steel Cord Belt and Splice Construction Modernizing their specifications, Improving Their Economics

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Steel Cord Belt and Splice ConstructionModernizing their specifications, Improving Their Economics Print

L.K. Nordell, USA

Summary

Belt conveyers constructed with steel cord tensile members are generally designed with a breaking strengthrating that exceeds the operating load by 6.7 times. This multiple, or ratio, is often referred to as the SafetyFactor (SF). This 6.7:1 SF has been chosen by belt manufacturers and industrial standards to represent thecriterion for strength which engineers must follow during design development This paper will demonstrate thattheir approach, developed over 30 years ago, is not practical today. It results in a significant economic penalty tothe owner. Belt and splice construction materials and modem analytic procedures can provide safe operationusing an SF ratio below 6:1 at breaking strength ratings from ST1000 N/mm to above ST6000 N/mm. Splicefatigue failure criteria and stress limits of the core rubber and cable are discussed. This method is used toresolve modern steel cord belt and splice design. The economic benefits can be substantial for new andreplacement belt.

1. Introduction

This paper is an extension of the authors 1991 and 1993 papers, [1] and [2] respectively. Further research isbeing conducted on the viscoelastic power equation, the hyperelastic splice modeling methods, and fatiguemechanics of steel cable and rubber. These criteria and other notable influences provide the necessaryrefinements to guide the changes in belt and splice specifications and construction methods. These tools cansupport the mine owners interest in ranking the performance of belts rubber properties for its demand power(viscoelasticity), belt wear resistance (toughness), and true temporal capacity (splice design and beltconstruction). All belts are not created equal.

Field investigation, together with modern theory end laboratory studies confirm! firstly, that the empirical powerequation [3] or [4] does not provide a reasonable level of accuracy [5], and, secondly, the belts strength cannotbe determined from elementary testing of cable breaking strength and rubber Hblock static pullout forces [2].These grey areas of understanding need a more accurate approach to fulfill the demands placed on the engineerfor today's larger and more costly conveying systems.

The belt Safety Factor (SF) will be developed from basic principles. Decomposition of the SF into itsfundamental elements leads to an understanding of how a safe and reliable method can be employed to quantifya realistic SF value. The decomposition is an expansion of the methods set forth in DIN 22101 [3].

Some owners, engineers and manufacturers have equated the static (pullout) strength capacity of the splicewith its dynamic, fatigue or endurance strength, claiming they can achieve a splice strength equal to 100% ofthe belt breaking strength. This reasoning is highly inaccurate as will be shown, Fig. 1 demonstrates thevariability of the rubbers stress intensity using FEM techniques. The stress is not homogeneous as some believe.

Fig. 1: Finite element model of splice 3step pattern (213) illustrating maximum stress intensity zones inrubber.

New splice designs reduce critical stresses yielding greater strength and reduced cost

2. Dollars and Sense

New and replacement belt, designed with standards appropriate for the application, can make significant profitfrom an engineered reduction in the 6.7:1 Safety Factor. Direct savings come from the reduction in belt strengthand belt weight affecting motor size, power demand, reducer selection, number and complexity of splices, pulleyassembly sizes, idler life, structural steel supports, civil works grading allowances, etc. The belt can be 25% ormore of the total conveyor investment. The difference in cost can result in millions of dollars in savings. Thegreatest savings are achieved with a relatively horizontal (slight undulation) belt profile. Conveyor Dynamics,Inc. (CDI) assisted in the design of the Channar 20 km overland conveyor in Australia [5], where the SF =5,0:1, but DIN 22101 was enforced by an engineering audit (Case 2 below). A relative belt cost savings tableshows a significant difference in conventional standards versus the recommended approach using Case 3 as thebasis.

Case Belt SF Remarks Cost Penalty

1 6.7:1 Designed per DIN 22101 39%

2 5.0:1 Designed per DIN 22101 23%

3 4.5:1 Recommended Design 0%

Note:



a. Case 1 is designed with conventional friction factors, and conventional published splice methods.b. Case 2 is designed with a slight reduction in conventional motion factors noted for superior installation

alignment and maintenance Channar Design.

c. Case 3 is designed to new power equation methods, and a belt specification and construction formulationcontrolled by the splicing system Channar and other measurements, since 1989, verify the potential.

3. Manufacturers Recommendation for Safety Factors in Steel Cord Belts

Some of the leading steel cord belt manufacturers were polled for their assessment on support of the reductionin the SF. Their comments are summarized here:

3.1 Bridgestone Corporation

Bridgestone Corporation completed a detailed investigation of the Belt Safety Factor in 1982. A finite elementmodel was developed to study the nonlinear rubber strain and fatigue behavior. Extensive testing was made onthe fatigue performance of the belts steel cords. These studies were produced in response to an inquiry for anST7000 N/mm class belt, Our investigation showed that this class of belt could be constructed with a BeltSafety Factor of 5.0:1 instead of the commonly used 7,0:1. Bridgestone has in service low Safety Factor beltwhich has been in operation for more than 10 years. Regular monitoring of its performance has providedunderstanding that the practice is sound.

We continue today, through further research and testing, to advocate improved Safety Factors to keep pace withadvancements being developed in materials and engineering methods, Dynamic fatigue testing of the belt and itssplice pattern as defined by DIN 22110 are also a necessity when recommending new classes of belt construction[6]. a course, we must bear in mind any reduction in the so called Safety Factor must also be accomplished withfull understanding of all engineering issues and design standards which must apply.

The Safety Factor is dependent on many elements outside the control of the manufacturer. When the ownerwishes a significant cost savings and is willing to participate in proper operation and maintenance, then greaterbenefits can be achieved. A Safety Factor closer to 4.0:1 may be offered.

The conveyor belts economics can have a large impact on the viability of mining projects. To these endsBridgestone is providing its full support redefining the criteria and standards that promote a more costeffective,powerefficient, and safer belt for the mining industry.

3.2 ContiTech Transportbandsysteme GmbH

The Safety Factors given in the old DIN standard DIN 22101 of Feb. 1942 were calculated with the maximumbelt tension Ti and the breaking strength of the full belt width. This rough method resulted in a 10:1 or higherbreaking to operating strength ratio. It resulted in an ultrasafe recommendation. This method of belt selectioncan even be found today mainly on the low end tension (fabric carcass) applications.

The Safety Factors have been reduced with the introduction of high strength EPcarcasses, steel cord belts, andimproved methods of calculating the actual stresses on a conveyor belt. This is possible due to the understandingof better belt material construction properties and methods of calculating the maximum and steadystate belttensions. These forces are specified for a maximum material loading in regular operation. This later developmentis a departure from specifying belt strength based on its static capacity. Instead now the dynamic strength andthree other major operating conditions noted in DIN 22101 Feb. 1982 are common practice. The lowest SafetyFactor named here is 4. 8:1.

This way of belt selection, which is generally practiced today, makes use of more knowledge of the beltconstruction and better calculation methods of the belt tensions. It allows for a more accurate and thereforemore economical and safer selection of the belt. Greater savings potential is still to come to the high end'application.

The conveyor is a dynamic thing, nothing is static in starting. stopping and steady running. In the early 1980s, atthe University of Hannover. a dynamic test method for conveyor belt splices was developed which allows now anevaluation of the belt and its splices with regard to the time strength behavior, Although this method has alreadybeen introduced in the DINstandard DIN 22110 (Parts, Sept. 1993) 61 up to now it has only been used by afew belt conveyor manufacturers, engineers, and users.

Knowledge of belt dynamic properties allows the engineer not only to develop the belt and the splice for betterperformance, but also to use a dynamic Safety Factor for the selection of belt and splice design. Today, thisdynamic Safety Factor (splice dynamic efficiency) may be specified at a ratio of 2:1 or less. This results in anequivalent 50% splice dynamic efficiency. In the future. it will be necessary to have more information about theactual strain history on the belt and a corresponding collective model for its operation over the required servicelife.

3.3 The Goodyear Tire & Rubber Company

The definition of a proper Safety Factor for steel cord belts has been discussed among belt manufacturers,engineering firms, conveyor designers, and customers for years. Typically a 6.67 Safety Factor is offered forgeneral practice and 8.0 for severe or abusive service. However with the advent of more sophisticatedmathematical and computer analysis there is a trend toward reduction in Safety Factor with 'properly' engineered



Systems.

Perhaps the greatest difficulty in defining the Safety Factor is the acceptance of a suitable reference criteria,some of which can be listed as follows:

a. Belt ultimate strength.

b. Splice strength through static shear analysis.

c. Splice strength generated on small scale cyclic laboratory test equipment to determine rubber fatiguecurves at tensions 23 times normal operation.

d. Splice strength determined by large scale two pulley dynamic test equipment operating at tensions 24times normal operation.

e. Splice strength determined by finite element analysis which combines laboratory data with computersimulation of static shear loading.

In truth, all of the above have merit but none should be considered alone as the "true" Safety Factor, Eachtechnique has benefits and provides information that can be used by the belt engineer in designing the correctsplice for the specific application.

The Safety Factor can be reduced using theoretical studies, when the total belt system has been analyzed for alltransient and loading conditions. This reduction in Safety Factor can offer the end user considerable savings incapital cost from the belt and other components.

However, Goodyear has been reluctant to recommend a reduction in Safety Factor beyond 6.67. In many cases,belt systems are designed for 1015 years of operating life before requiring replacement. When the originaloperating tensions are calculated, the ideal Motion and loading are inputted. Practice] experience has shownmaintenance conditions can deteriorate and that mine tonnage requirements can be increased beyond originalexpectations.

Also it is common for accidents to occur such that a large number of cords in the body of the belt are broken ordamaged. A reduced Safety Factor, based totally on splice strength, makes no provisions for these conditionsand could result in lower tonnage rates until replacement belt is acquired. With 6.67 and higher Safety Factor itis not uncommon to have up to 10% of the cords damaged or broken without consideration to reduced tonnages.

Although a reduction in belt Safety Factor below 6.0 is possible and has been supplied by Goodyear. it shouldonly be considered after careful analysis of future mine maintenance conditions, possible damage to the beltthrough operations, and risk analysis of cost due to mine downtime.

Author's Note:

The dynamic SF for hoisting ropes begins at 5:1 [7] [8]. These ropes are not impregnated with rubber and havea sheave to rope diameter ratio usually below 50:1. The belt high tension pulley diameter to cord diameter ratiois usually at 100:1 or larger. The most significant factor is that the belt cover protects the belt's steel cord,whereas the hoist rope bends around bare steel sheaves.

4. Splice Structural Definition

Splice patterns shown in Fig. 2 are defined by the number of steps and step arrangement. They are made up ofseven fundamental dimensions:

d effective cable diameter given by cable manufacturerp cable pitch, the distance between cable centers in beltg rubber gap, the closest distance between cable diameters in splicee distance between cable endsb cable bending zone transition between splice end and its stepsS step length, proportion of each cable that ends in splice in relation to splice length excluding bend zonesL splice length, total length of cables passing including e, b, and a (bend zones are excluded for this paper).



Fig. 2: One and two step splice arrangements with dimensional variables section through horizontal axis

The numbered black zones illustrate the rubber panels in shear from opposing cables. We refer to this as theactive shear stress zone. Historically, this method has been used by manufacturers to calculate the splicestrength (i.e. more panels and their total length for a given step pattern equate to greater strength). This is nottrue for more than the single step splice. The nonblack zones, in the two step, also contribute.

The white unmarked areas are called the passive shear stress zones, where the cables are pulling together in thesame direction. They also transmit shear forces due to differential elongation between cables. Comparing Fig. 1to Fig. 14 illustrates that the white areas can transmit a significant force and cannot be neglected in the spliceanalysis. Fig. 9 illustrates typical rubber loading in each of the shear panels between cables. This also showssignificant response in the passive shear panels.

5. Safety Factor Definition

The term Safety Factor, used by belt manufacturers and industrial standards, does not imply any value on themargin of safety available beyond the design operating load, In its simplest form, the value of the Safety Factor(SF) is equal to the total belt cable breaking strength (not splice), divided by the belt design operating loadunder ideal conditions.

SF = static break strength / design load (1)

The belt static strength or ultimate strength is not an appropriate design criterion for dynamic operation. Boththe cable and rubber undergo structural damage when subjected to repeated cyclic steadystate and dynamictransient loads. The rate of structural damage is defined by each material's fatigue or endurance strengthrelative to the number of load cycles [9] [10] [11] [12] [13] [14]. The belt's steel cable infinite life cycles canonly be guaranteed when the peak cyclic load is kept below 25% of the cable's breaking strength. Rubbercontinues to degrade with cyclic loading, as the polymer's molecular chains breakdown (scission) from repeatedstrain even under relatively light forces. The life cycles are exponentially dependent on the applied force (stress)as shown in Fig. 3 and its phenomenological equation:

f (life cycles) = α · ex (2)

Fig. 3: Splice endurance and belt safety factor curve

where x = (β · force + δ)η and α, β, δ and η are functional constants depending on rubber compound, cableconstruction, and splice pattern, including axial and bending stress history. Delta (δ) equals infinite lifeparameter of rubber or steel cord.

The fatigue decay rate and endurance curve stress magnitude can be significantly improved with special rubbercompounding and additives. Belt manufacturers have developed special rubber compounds which yield superiorfatigue and adhesion properties while also resisting environmental attack.

The Safety Factor can be decomposed into five major independent subclassifications, four of which are shown inFig. 3. Decomposition provides assessment of the individual contributions of each factor.

1. Splice Fatigue (Rubber & Steel Materials, Pattern; Load)2. Elongation (Bending, Flexure, Transitions, Curves)3. Degradation (Damage, Errors and Omissions)4. Dynamic Loading (Starting and Stopping)5. SteadyState Loading (Running).

Each of the subclassifications also has many factors contributing to their allowable stress or strain ranges. Inaddition, the individual subclass items have tolerances on their stress ranges which must be rationalized.

5.1 Splice Fatigue Allowance

The splice strength can vary with its materials and methods of construction. Some of the main features include:



Fig. 4: Splice fatigue strength vs.load cycles on rubber and steel

Fig. 5: Splice strength vs. optimizedstep length (normalized length is

dimensionless)

a. Rubber and steel properties (special compounds which resist fatigue, special cable designs) Fig. 4b. Rubber core gap (g) increasing g reduces rubber stress and increases steel cord stress per Fig. 5.

Normalized refers to the gap dimensionless property.

c. Splice step number, 1, 2, 3, 4 (high steps are less efficient. due to the increase in stress concentration intheir rubber and steel)

d. Splice pattern step sequences impose different stresses on rubber and cables, as shown in Fig. 14 andnoted in 6.1c [2] [12].

e. Splice length 95% of the strength can be achieved from 50% of the beneficial length. Usually it islarger than belt manufacturers' recommendations, Fig. 13.

5.1.1 Splice Core Rubber Gap

The splice core rubber and its gap specifications are probably the single most important feature of the splice. Itis the hardest dimension to control with any accuracy, and has the most profound effect on the usable beltstrength when not sized or constructed correctly.

Manufacturers and standards have published recommendations for the core rubber gap. Fig. 6 illustrates the corerubber gap of three major manufacturers, the standard DIN 22131 [16], and their mean over the range of steelcord STratings. Regardless of the strength rating, the value of g equals about 2.5 mm, Hblock pullout testingdemonstrates that significant improvement is realized by equating the core rubber gap to the cable diameter asshown in Fig. 7. Using 001's FEM pullout values which are dependent on the g/d ratio in Fig. 7, the pulloutstrength of the splice is plotted in Fig. 8. Fig. 8 clearly demonstrates that these practices seriously underrate thesplice potential and reduce the belt's economy . We recommend the optimization concept proposed in Fig. 5.

Fig. 6: Core rubber gap distance vs.published STratings

Fig. 7: Hblock pullout force vs.rubber gap to diameter ratio (g/d)

Fig. 8: Pullout force per Fig. 7 vs. published STratings and alternative

Core rubber endurance properties cannot be inferred from the Hblock pullout test results [14]. Static pullouttesting can only provide an estimate of cable adhesion.

A number of different FEM stress plots, Figs. 911, are provided to show the sensitive regions and themagnitudes of change. The color gradients are shaded in 19 hues illustrating the maximum to minimum stress,or from a positive to negative extreme. The stress plots were developed from the 3step pattern shown in Fig.14.



Fig. 9: FEM axial shear stressgradient in 3step splice pattern

(213)

Fig. 10: FEM torsional shear stressgradient in 3step splice pattern

(213)

Fig. 11: FEM tensile stress gradient in 3step splice pattern (213)

The core rubber is loaded in axial shear, transverse shear (torsion). and in tension. Fig. 1 illustrates the rubbersmaximum stress intensity for the typical 3step, 213 pattern. Figs. 911 illustrate the FEM results on themagnitude of axial shear stress, torsional shear stress, and tensile stress. Torsional stress represents 1015% ofthe axial shear. The torsional stress comes from the cables' rotation which is coupled to its axial extensionthrough the helical arrangement of the wire groups in the cable, and through twisting when bent over pulleys.

Accuracy of the FEM analytic techniques have been verified by testing of actual splice designs on the HannoverUniversity machine [6] and by laboratory dynamic testing. CDI designed a special jig to test the fatiguecharacteristics of rubber. Testing, with the jig, was carried cut by a leading belt manufacturer. The results equalthe hundreds of dynamic tests conducted at Hannover as reported in the thesis of H. VAN DER WROGE [12]. Hecalculated that when the belt achieved 10,000 load cycles, with all splice patterns and manufacturers considered,the shear stress equaled 2.2 mPa + 15% The CDI test jig was used on one rubber compound. Twenty four testswere conducted, at differing core gaps, and a range of forces that produced one fatigue failure curve for therubber. The number of load cycles ranged from less than 1,000 to more than 30.000 repetitions. The shear stresscurve, at 10,000 load cycles, equaled 2.19 mPa 10%. The rubber fatigue failure curves of VAN DER WROGE,CDI, and HAGER [14] obey Eq. 2.

5.1.2 Splice Step Length

Forces are transmitted between cables by a shear panel which. in simple terms, is described by a width, height,and length. The core rubber gap (g) approximates the width, the cable diameter (d), the height. and the stepdimension (s), the length.

The step length selection depends on cable diameter, cable construction, splice step sequence, rubber properties,and proprietary practices of some manufacturers [15].

Three belt strengths were selected from the DIN Standards 22129 and 22131 to illustrate how this standard hasspecified the shear panel. The dimensional comparisons are made in proportion to their respective diameters asshown in Fig. 12. A greater gap (g) and greater step length (s) each reduce the rubber's shear stress. Whencomparing the three shear panels in proportion to their cable diameters, the higher strength belts have smallgaps. lengths and shear panels to carry the force. Their optimization is suspect.

Fig. 12: Splice step, cable and core rubber sizes proportional to their respective diameters

Fig. 13 illustrates the general form of the CDI optimized step length, where the step length is defined as thedimension of the critical cyclic shear stress zone. 95% of the step length strength is achieved in 50% of theoptimum length. This is larger than is normally recommended.

Fig. 13: Optimum splice strength vs. core rubber gap (normalized gap is dimensionless)

Some manufacturers continue to cite the shear load capacity, splice pattern, and step length based upon Hblock



static experimental measurements. Fig. 4 shows that the Hblock static pullout strength is clearly notrepresentative of the splice endurance strength.

Fig. 14 illustrates the 3step configuration and step length formulations for three typical, frequentlyrecommended patterns. The pattern sequence numbers, 1, 2, 3, denote the respective order and length eachcable is embedded in the splice proportional to the maximum step length for one repeating pattern. The 123step sequence is the DIN 22131 recommendation for surface conveying. The 132 pattern is the DIN 22129pattern for underground service [17]. The 213 pattern is recognized by some belt manufacturers as superior tothe other two patterns. Comparing these three patterns, using FEM, we find the peak rubber and cable loads are:

Splice PatternMax. Rubber Shear Max. Cable Force

σmax/σnom Fmax/Fnom

123 1.43 1.17

132 1.25 1.27

213 1.22 1.25

Note: These results are for one cable diameter and its construction, as recommended by a belt manufacturerwithout regard to optimization.

Fig. 14: Standard 3step patterns. Three alternatives and their stresses

There are other 3step patterns that provide superior benefits. Proprietary patterns, cable designs, core gaparrangements, reinforcing techniques, and rubber compounds all reduce the maximum to nominal load ratioscloser to 1:1.

5.2 Degradation and Elongation Endurance Forces

Degradation and elongation forces are additive to the running force and set the minimum endurance limit ofsplice strength. They are rarely quantified by the engineer.

Elongation refers to a class of forces or stresses which produce additional strain or elongation, such as; idlertransition shape change to high tension head pulley; convex vertical belt curve edge stresses in the high tensionregion; bending of the belt over pulleys, causing added torsional strain on the rubber from cable twist; turnovers,belt alignment, etc. They can be calculated. DIN 22101 allows 100% of the running tension for the many possibleforces that are omitted from standard calculation methods.

The significant values are: % Running Tension

1. Idler transition (FEM studies) ≤15%

2. Belt alignment(using CEMA and DIN 5% allowance) ≤15%

3. Vertical curves (not generally applicable) ≤10%

4. Bending over pulleys ≤10%

Total ≤50%

Degradation allowance accounts for general damage caused by:

splice construction errors

belt age

rock impact

transition idler alignment errors

pulley alignment errors

pulley diameter shape errors



cover wear transverse variations

drive load sharing anomalies

surface buildup on pulley and belt

proximity of pulleys to each other

fixed takeup & high takeup travel hysteresis

pulley lagging shape errors

general cable breakage (rock entrapment, etc.).

The present standard allows for elongation, but does not discuss degradation. It is implied without detail.Elongation and degradation strength factors exceed 100% of the design running force. Degradation alone exceeds50%. The operator can make significant contribution to lowering his capital cost by practicing a goodmaintenance policy.

Field constructed splices, we believe, contribute at least 25% of this allowance. Our inspection of many splice Xray patterns throughout the world, leads us to believe that only a few vulcanizing contractors and plant crewshave the technical knowledge to properly construct a splice. Many errors are made without their knowledge.Often the construction errors reduce the effective strength by half, causing splice failures to occur within one ortwo months. In engineering terms, the splice construction errors can reduce the belt strength by a factor equalto, or exceeding, the belt material transport design load. They can be avoided with proper training in the splicefunctions and making certain the splice crew have knowledge of the calibration and maintenance of thevulcanizing machine.

We recommend the splice construction degradation allowance be increased to 60% of the running tension unlessspecial methods are used to control fabrication. This loss is made up by the reduction in the dynamic allowancesection.

No benefit in splice technology can be achieved unless the splicing contractor is aware of the importance of hisfunction and that he can meet the specifications of proper splice procedures.

Figs. 15 and 16 show some good and bad techniques. These figures are Xray patterns taken on conveyors ofsignificant capital.

Fig. 15: Xray of bad splice technique. One cable carries no load(8), and small gaps (8, et al.), large gaps (7 & 9) and poor cable

alignment is evident

Fig. 16: Xray of good splice technique

5.3 Dynamic Analysis (Starting & Stopping)

DIN 22101 calls for starting and stopping strength allowance equal to 40% of the running force (favorableoperation).

% Start = [ SF (Starting)1/SF (Running)1 1] x 100

= [0.209/0.149 1] x 100 (Ref. Fig. 3) = 40% (4)

CDI has different criteria for dynamic forces which are beyond the scope of this paper. When high momentaryloads are applied, and are frequently repeated, then special endurance or material yielding stress limits must beevaluated.



A simple analogy shows the effect of momentary starting.

Assume the following:

1. 40% start overload per DIN 221012. 1 minute per start per 1,000 m transport distance3. 10 starts per day4. 1,440 minutes per day operation5. Belt speed 5 m/s6. Damage is simple exponential function.

Starting influence on SF (dynamic):

SF (dynamic) = exp No/Ax, whereNo. = 10,000 load cycles and SF valuesAx = No[1/In(Run) + γ/In(Start)]

Start = 1.40 x run = (1.40)(0.2985) = 0.4179γ = ratio start to run cycles per day = 10/216 = 0.0463

SF (dynamic) = 0.321 with respect to run = 15% with respect to running SF = 6.7:1

= (1.15 x 0.1492)1 = (0.1716)1 = 5.8

SF (dynamic) = 5.8:1, not 4.8:1 as in DIN 22101.

Therefore, the dynamic forces do not contribute greatly to the overall endurance strength allowance for mostconveyor systems.

Today. CDI provides a 15% strength allowance over the running force for dynamic forces imposed on the splice,not the DIN 22101 40%.

5.4 Power Equation

This section is included to show that significant errors can be made in the assessment of belt tensions based onstandard power equations. This error becomes exaggerated when the belt strength is specified. Using the rule SF= 6.7:1, the cost increases dramatically.

The conveyor's main rolling resistance is produced by four major and three minor factors:

Major:

1. Idler indention into the belt cover rubber2. Belt flexure over the idler trough and between idlers3. Material agitation or trampling together with belt flexure4. Idler bearing and seal drag.

Minor

5. Drive assembly efficiency losses6. Flexure of the belt over pulleys7. Material loading acceleration turbulence and skirt drag.

Idler indention (deformation) accounts for 6080% of these losses depending on the rubber's viscoelasticproperties. Viscoelasticity is the term which defines the hysteretic loss that occurs in the rubber duringdeformation.



ISO 5048 [18], DIN 22101 [4], and CEMA [3] and a number of handbooks suggest various friction coefficients todescribe the rolling resistance. They are all subjective. Some only require the dead weight of the belt per unitlength, others add idler spacing, belt tension, and ambient temperature and sometimes a constant. Many factorsgo unaccounted for, such as: idler diameter, idler trough shape, rubber cover thickness, rubber compound, beltstiffness, material load profile, vertical and horizontal curve forces, idler alignment, material and belt center toidler axis, etc. Standard methods, as noted above, are used because they are simple. With small inplantconveyors, and conveyors with high lift to length ratios, this may be acceptable. With larger overland conveyors,gross errors can be made.

A set of constitutive equations was first proposed by SPAANS [19] that described the indention hysteresis loop,in rubber, under deformation by the idler in the manner shown in Fig. 17. These equations have continued to berefined as researchers realized their importance [20] [21] [22].

Fig. 17: Deformation of rubber by idler indentation (static case)

Three years ago, CDI was awarded a contract, by Syncrude Canada Ltd., to develop a modern viscoelasticitypower equation [23]. This work is now essentially complete. The modem power equation has an accuracy ofabout 5% of measured condition when all basic information is known. We have been fortunate to assist in thedesign and commissioning of a number of large projects where accurate field measurements were made:

a. Kennecotts Bingham Canyon UCD Expansion 1987 (5.5 mile overland, transporting 10,000 st/h).b. Palabora's Slope Belt 1988 (9,600 hp: 7.1 MW).c. Channar 20km Overland 1987(2,100kW on each of two flights).d. Phelps Dodge Morenci 1992(6,000 hp downhill).e. Syncrude Canada Ltd. Present (5,000 hp cold weather (40C) analysis and measurements only).

The field measurement accuracy confirms the theoretical methods. In addition, extensive laboratory work hasbeen conducted on products at full scale for belt, idlers, loading, speed and temperature, Fig. 18 demonstratesthe temperature tonnage sensitivity to rubber viscoelastic losses. Note. the friction does not continue to increasewith temperature below 20C for this compound. At 30C, the friction loss is equal to 20C. The idler bearinglosses were not included.

Fig. 18: Rubber hysteresis multiple vs. temperature at three tonnages rubber belt on rubber covered idlers

The rubber compound can have a profound influence on power. Fig. 19 illustrates four compound differences. Inthe extreme, compound A has 4 times the hysteresis drag as the base compound. This equates to more thandoubling of conveyor power for a horizontal system.

Fig. 19: Rubber hysteresis vs. temperature for four compounds

6. Conclusion



Specifications for belt strength are highly dependent on proper assessment of splice design and constructionintegrity, and on the accurate analysis of conveyor power. Significant differences exist today between the wellrecognized industrial standards and modern engineering principles. The economic impact to the mine ownersshould riot be ignored by the manufacturers. Some, as noted in this paper, are making the necessary changes tooffer products which fit today's welltested technology. Hopefully, this paper will help to bring the industrytogether and upgrade the standards.

Acknowledgments

The author wishes to express his appreciation to Bridgestone Corporation, ContiTech TransportbandsystemeGmbH, and The Goodyear Tire & Rubber Company for sharing their comments on the belt Safety Factor.

The research for this paper could not have been carried out without the most able support of Dr. XIANGJUN(JOHN) QIU, CDI's Manager, Applied Mechanics.

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17. German Industrial Standard, 1988. Steel cord conveyor belts for underground coal mining, dimensions,requirements, DIN 22129.

18. International Standard ISO 5048. Continuous Mechanical Handling Equipment, 1st Edition. 1979.19. SPAANS, C.: The Indention Resistance of Belt Conveyers: Delft Technical University, The Netherlands,

Dept. of Mech, Eng. WTHD No. 103. Jan, 1978.

20. JONKERS, C.: The Indention Rolling Resistance of Belt Conveyers: Frdern und Heben. Vol. 30. pp. 31231 8. 1980.

21. LIMBERG, H.: Untersuchung der trumbezogenen Bewegungswiderstnde von Gurtfrderanlagen:Dissertation. Technical University Hannover, Germany. 1988.

22. GREUNE, A.: Energiesparende Auslegung von Gurtfrderanlagen: Dissertation, TU Hannover, Germany,1989.

23. MELLEY, R., BLAND, S., and MCTURK, J.: Optimization of Oil Sands Conveying through FieldMeasurements: Paper presented at AIMESME Annual Convention, Reno, Nevada, USA. February 1993



Mr. L.K. Nordell, President, Conveyor Dynamics Inc. (CDI),1111 West Holly Street, Bellingham, WA 98225, USA.

Tel.: +1 206 671 2200; Fax: +1 206 671 8450.

Documents

Steel Cord Belt and Splice Construction Modernizing Their Specifications, Improving Their Economics